2009 prerunner vibration problem

Looking for some advice here because I haven't been able to pinpoint exactly what's going on. I have a 2009 prerunner and just hit about 88k miles. it recently started having this vibration/shimmy as i get to about 1500 rpm and it's just on the verge of shifting up, and it does it between 2nd and 3rd, 3rd and 4th, and 4th and 5th as far as i can tell. if i step on the gas significantly and accelerate more, it can get past the vibration. it's exaggerated and does it way worse whenever i'm going up an incline.

I got under the truck and checked u-joints and the carrier bearing and nothing seemed out of the ordinary to me. I don't think transmission mounts seem irregular either.

Today I tried putting the truck in 4th while driving and taking overdrive out of it, and it seemed to eliminate the problem. This is the one factor that seems to make it a little different than a bad u-joint or carrier bearing.

Is it possible that a transmission flush is really what is needed? I've seen so many different "fixes" out there that I really don't know what to believe at this point.

Any help is appreciated!